1. Field of the Invention
The invention generally relates to photovoltaic devices and more particularly to an improved silicon solar cell having a shallow N-type region and a contact characterized by reduced contact resistance.
2. Description of the Prior Art:
As pointed out in United States Letters Pat. No. 4,029,518, in general, a solar cell comprises a semiconductor wafer of one conductivity type having diffused into one surface thereof a thin layer of a conductivity type opposed to that of the semiconductor wafer. Where the thin layer comprises an N-type region, the region is formed by diffusing impurities into the semiconductor wafer to establish an N on P junction between the layer and the wafer. Diffusion often is accomplished by treating a surface of a silicon wafer with POCl.sub.3 and thereafter heating to a temperature of approximately 850.degree. C., the duration of the period for heating being determinative of junction depth. As is well known, the opto-electric conversion efficiency of a solar cell is enhanced as its N-type region becomes thinner. Silicon solar cells presently are composed of shallow N-type regions of 0.5 .mu.m, diffused into P-type silicon.
To provide a low cost method of collecting currents from the N-type region of a single crystal silicon wafer, techniques requiring a use of screen printing and fired metallizing steps frequently are employed. Conventionally, commercially available metallizing inks which contain finely divided glass frit in addition to metal powder and organic vehicles are employed for depositing contacts on the surface of the N-type region. The metal powder frequently includes silver, aluminum, nickel, gold, copper or alloys of these with precious metals including platinum and palladium.
Samples of silver, aluminum and nickel ink fired onto silicon wafers of varying carrier concentrations, crystallographic orientations and dopant types normally can be expected to be found deficient in specific contact resistance and/or metallization penetration. For acceptable solar cell operation, the specific contact resistance must be no higher than 2.times.10.sup.-2 .OMEGA.-cm.sup.2 and metallization penetration into the shallow N-type region must be considerably less than 0.5 .mu.m in order to avoid a shorting of the cell.
As is known, specific contact resistance can be kept below the required value by firing commercial materials, of the type aforementioned, at 700.degree. C. for about ten minutes in order to effect suitable sintering of the screen printed contact. Unfortunately, heating of the cells to such temperatures for such periods tends to cause the metallization penetration to exceed 0.5 .mu.m.
Since conventional metallizations contain frits which are designed for application to ceramic and glass surfaces, the electrical properties of the frit constituents generally are not considered in formulating frit for the inks. Hence, it is not uncommon to find frits containing both group III and group V dopants. Consequently, the ability of these frits to locally increase carrier concentrations at the surface of the N-type region and to reduce specific contact resistance is severely impaired.
It is, therefore, the general purpose of the instant invention to provide an improved solar cell characterized by a semiconductor wafer of P-type material having a shallow N-type region diffused into the P-type material for establishing a P-N junction between the region and the wafer and an improved contact affixed to the N-type region and which serves to establish a zone of increased carrier concentration.